The transition from an aqueous medium, in which the ancestral Charophyceae group lived, to a gaseous medium, exposed the early land plants to new physical conditions.
For instance, in place of the structural support of unlimited water the first land plants in an aerial environment faced desiccation exposure and the compressive effects of gravity. The early thalli would have also been exposed to relatively higher photon flux density, which would previously have been exponentially attenuated by a water column, and a 104 gain in diffusion rate of CO2 as water places such diffusive limits (Osmund et al 1982).
Consequently, key physiological and structural adaptations, over time, needed to occur in early land plants. Development of cutins to reduce water loss probably evolved from pre-existing elements of the primary metabolism in the ancestral charophycean algae (REF). Additionally, photoprotective mechanisms possibly developed to cope with higher photon fluxes in aerial environments utilizing aspects of the already existing photo respiratory pathways (Kendrick and Crane 1997). However it is unknown whether the photoprotective role of photorespiration evolved in photosynthetic organisms in shallow water at high light intensity or in the early land plants.